1. Field of the Invention
This invention relates to the recovery of oil from a subterranean formation. More specifically, this invention relates to the recovery of oil from subterranean formations by injecting thereinto liquids containing both viscosifiers and microemulsions.
2. Description of the Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original oil in place in a reservoir is expelled by natural mechanisms. It is also well-known that conventional methods of supplementing natural recovery are relatively inefficient. Typically, a reservoir may retain half its original oil even after the application of currently available methods of secondary recovery. Accordingly, there is a continuing need for improved recovery methods which will substantially increase the ultimate yield of petroleum from subterranean reservoirs.
Waterflooding is by far the most widely practiced method for recovering oil from a formation after naturally occurring forces in the formation have declined in their ability to expel oil. In waterflooding, water is injected through an input well to drive oil to offset producing wells. Much of the current work in oil recovery technology has been directed toward improving the efficiency of waterflooding processes or developing alternative processes.
Surface-active agents or surfactants are one class of materials which have been proposed for improving the efficiency of waterflooding processes. Much of the oil that is retained in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir. Because the normal interfacial tension between the reservoir oil and water is so high, these discrete droplets are unable to sufficiently deform to pass through narrow constrictions in the pore channels. When surface-active agents are added to the flood water, they lower the interfacial tension between the water and the reservoir oil and permit the oil droplets to deform and flow with the flood water. It is generally conceded that the interfacial tension between the flood water and the reservoir oil must be reduced to less than 0.1 dyne/cm for additional recovery.
One method for reducing interfacial tensions and increasing oil recovery is through the use of microemulsions. Microemulsions are thermodynamically stable, transparent or translucent mixtures of a liquid hydrocarbon, water and a surfactant. Optionally, a co-solvent such as alcohol and electrolytes may be present in the mixture. Generally, microemulsions may be oil-external, water-external or microemulsions wherein no external phase can be identified. In practice of microemulsions slug is typically injected into the formation, followed by an aqueous slug thickened with a polymer (a mobility buffer) which are both driven towards a producing well by injecting water or brine.
In a departure from conventional microemulsion flooding, wherein the microemulsion is miscible with the formation fluids at least prior to injection, it has been suggested in U.S. Pat. No. 3,885,628 (Reed, et al) to inject microemulsion systems which are initially immiscible with formation crude oil and formation water. This patent also suggests injecting two or more mutually immiscible phases into the oil-bearing formations to recover oil. The different phases preferably have physical and chemical properties broadly approximating those of the formation fluids. Although this approach to microemulsion flooding appears to have promise in recovering oil, surfactant and polymer adsorption and retention continue to reduce displacement efficiency.
In U.S. Pat. No. 4,240,504 (R. L. Reed), a microemulsion phase is simultaneously injected into a formation with an immiscible aqueous phase at controlled rates and viscosities. Under the specified conditions, the microemulsion phase is believed to preferentially displace crude oil while the immiscible aqueous phase preferentially displaces brine, thereby increasing oil displacement efficiency.
In U.S. Pat. No. 4,125,156 (Glinsmann) an enhanced oil recovery method is disclosed which uses an aqueous surfactant system which is capable of forming a multiphase microemulsion system in situ upon injection into the formation, which microemulsion when complete is immiscible with the reservoir crude.
Unfortunately, the very low interfacial tension which exists between a microemulsion and an oil/water bank means negligible capillary pressure and relative permeabilities nearly proportional to phase saturations. Under these circumstances the mobility of the microemulsion increases in the transition zone between the oil/water bank and the microemulsion, tending to promote fingering. As the microemulsion sweeps out the oil from a portion of the reservoir, the effective permeability of the reservoir increases, promoting flow through the swept region, and inherently encouraging fluids to finger and bypass oil. For this reason, prior methods suggest the addition of a mobility control polymer or viscosifying agent to the microemulsion or other injected phases. However, this creates a new set of problems. Often, not enough polymer will dissolve or disperse in higher salinity microemulsions or other phases to give good mobility control. Further, bank deterioration upon flooding often results in repartitioning of polymer resulting in the break-out of gel-like aqueous phases or viscous emulsions, and causes increased surfactant retention. This problem of polymer partitioning and consequent plugging, etc., generally becomes worse upon the formation of multiple phases in a reservoir during flooding. For example, an indication of phase instability upon polymer addition to an ordinarily optimum injection composition is evident in previously cited U.S. Pat. No. 4,125,156, Column 22, Table VIII (see footnote "d"). Some of these problems and possible reasons therefor are discussed by E. I. Sandvik and J. M. Maerker in "Application of Xanthan Gum for Enhanced Oil Recovery," ACS Symposium Series, No. 45, Extracellular Microbial Polysaccharides, edited by P. A. Sanford and A. Laskin, p. 242 (1977).
Changes in phase character and phase behavior at the front and back of the microemulsion bank due to dilution also generally tend to increase surfactant retention, e.g. by increasing phase trapping. Some of the residual oil may not be accessible to the microemulsion until surfactant concentration has become too low for mobilization. There are other reasons that relate to cost and mobility control. To reduce the cost of a given process will require more efficient surfactant use, i.e., increased oil recovery per unit weight of surfactant injected. Surfactant contained in a given flooding medium is wasted through adsorption, phase trapping and mixing with resident brine and polymer drive water to the point where its concentration is so low that it becomes ineffective. When surfactant resides in a phase having an aqueous character, it flows preferentially through smaller pores where surface area per unit volume is high, and so is adsorption.
A need, therefore, exists for an enhanced oil recovery process which utilizes fluids giving the requisite interfacial tension reduction with minimal surfactant retention, capable of incorporating sufficient quantities of a polymer to give good mobility control.